The present invention relates to a method for calibrating the frequency of an RF oscillator in a mobile part of a mobile communications device when the mobile part of the mobile communications device is first switched on, having the following steps: sampling a modulated carrier that is transmitted by a base station and has a prescribed frequency accuracy; determining the frequency error between the frequency of the RF oscillator in the mobile part of the mobile communications device and a known channel frequency of the base station; and synchronizing of the mobile communications device with the modulated carrier.
Such a method is disclosed in EP 0 735 675 A2, the determination of the sequence error being performed successively by virtue of the fact that the search range for the frequency of the base station is expanded until the frequency of the base station is found. The channel frequency of the base station is certainly fundamental, but not known on the part of the mobile communications device.
Depending on the transmission method, the field of mobile communication is generally covered by a certain standard that requires a prescribed frequency accuracy for a base station of a communications device during transmission and prescribes a frequency accuracy for the case of reception in the mobile receiver part via further system properties.
This standard has been ensured to date by calibrating the RF oscillator during fabrication of the mobile part of a mobile communications device. However, calibration in fabrication is very expensive and requires a long production time.
It also has been proposed, for the purpose of correcting the temperature dependence of an RF oscillator in a mobile part of a mobile communications device, to make use of a temperature sensor that compensates the temperature-dependent error of the RF oscillator in conjunction with an appropriate algorithm. This method is also very expensive.
It is, therefore, an object of the present invention to propose a method by which the production costs for a mobile communications device are lowered and, at the same time, a reliable calibration of the frequency of the RF oscillator of the mobile part of the mobile communications device is ensured.